Solder masks are applied in soldering processes for various purposes including delineation of areas to be soldered, protection of copper conductors against corrosion and retention of electrical insulation between conductors. Conventionally, solder masks have been prepared by screen printing of thermosetting resins typified by epoxy resins. However, as the line width required of circuit patterns is decreasing, satisfactory levels of precision are no longer attainable by the screen-printing method and it has become necessary to adopt a photomechanical process in order to obtain fine patterns. In addition, with any improvement in wiring density, even higher reliability is required of the insulation between conductors and the requirements for satisfying this need have become increasingly regorous. It is difficult to form a thick coating by a single application of the screen-printing method and uniform coating on a substrate is also difficult to achieve. These problems could be solved by applying multiple coatings but the precision of printing decreases as the number of coating cycles increases and this also causes the problem of increasing the complexity of the relevant process steps. For these reasons, a need has arisen for the development of a light-sensitive film suitable for use in the formation of solder masks.
Photoresists, also called light-sensitive elements, are used in the formation of conductor patterns in printed circuits. However, they are not suitable for use in the formation of solder masks or protective films for chemical plating since their ability to resist heat, chemicals, etc. is not high enough to warrant use in these applications.
In order to solve these problems, light-sensitive elements adapted for use in the formation of solder masks have been proposed and they include, for example, the method described in U.S. Pat. No. 4,278,752 which relates to a light-sensitive element that is rendered flame-retardant, and the methods described in Japanese Patent Publication Nos. 59-23723 and 60-1885 which both relate to a composition having improved resistance to thermal shocks.
The light-sensitive elements described in these patents successfully attain the intended objects, i.e., heat resistance or resistance to thermal shocks, but they will experience the following problems if used as resists for chemical plating.
The conventional subtractive method of forming circuit patterns is being replaced by the additive method which is capable of forming wired copper patterns in selective areas without relying upon electrolysis. This additive method offers many advantages including the absence of any need for etching copper foils and treatment of the effluent, cost reduction, and increased reliability of through-holes, in particular small (.ltoreq.0.4 mm) ones. Because of these advantages, the active method is gaining popularity these days. However, the chemical plating bath employed in this method is strongly alkaline (pH ranging from 12 to 13 at 20.degree. C.) and involves treatment at elevated temperature of 60.degree.-80.degree. C. The light-sensitive elements proposed in the patents noted above are unsuitable for use under such hostile conditions.
Japanese Patent Public Disclosure No. 60-240715 describes a composition which was the ability to resist electroplating. However, with this composition, patterns are formed using an alkali developer, so it essentially has no resistance to strongly alkaline chemical plating baths.
In order to exploit the many advantages of the additive method described above, various versions of this method are under review and they include: the "full additive" method which consists of making through-holes in a substrate or catalyst-loaded substrate, performing an activation treatment on the entire surface of the substrate, printing a plate protective film on the substrate, and imparting the necessary conductor patterns solely by use of an electroless copper plating method; the "semi-additive" method which consists of printing a plate protective film by applying an electroless copper plate over the entire surface of a substrate, forming the necessary conductor patterns by electroplating, and removing the electroless copper plate from all areas of the protective film except those where the conductor patterns have been formed; and the "partly additive" method which employs a copper clad laminate and in which areas where through-holes are to be made are formed by an electroless copper plating method.
A composition that has high resistance to heat and chemicals and which hence is suitable for use as a protective film in these versions of the active method has already been proposed as shown in Japanese Patent Publication No. 61-47182. However, this composition lacks light sensitivity and finds only limited use since it is not amendable to photomechanical processes and is incapable of meeting the need for producing fine circuit patterns.
The present invention has been accomplished in order to solve all of the problems described above. An object, therefore, of the present invention is to provide a light-sensitive resin composition and a light-sensitive element that have high resistance to both heat and chemicals and which are particularly suitable for use in the formation of protective coatings for chemical plating. Another object of the present invention is to provide a light-sensitive resin composition and a light-sensitive resin composition and a light-sensitive element that have the added feature of improved resistance to the heat of soldering and/or improved adhesion to substrates.